Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory devices (e.g., NAND, NOR, etc.) have developed into a popular source of non-volatile memory for a wide range of electronic applications. Non-volatile memory is memory that can retain its data values for some extended period without the application of power. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Changes in threshold voltage of the memory cells, through programming (which is sometimes referred to as writing) of charge-storage structures (e.g., floating gates or charge traps) or other physical phenomena (e.g., phase change or polarization), determine the data value of each memory cell. Common uses for flash memory and other non-volatile memory may include personal computers, personal digital assistants (PDAs), digital cameras, digital media players, digital recorders, games, appliances, vehicles, wireless devices, mobile telephones, and removable memory modules, and the uses for non-volatile memory continue to expand.
In a NOR flash architecture, a column of memory cells are coupled in parallel with each memory cell coupled to a data line, such as a bit line. A “column” refers to a group of memory cells that are commonly coupled to a local data line, such as a local bit line. It does not require any particular orientation or linear relationship, but instead refers to the logical relationship between memory cell and data line.
Typically, the array of memory cells for NAND flash memory devices is arranged such that the control gate of each memory cell of a row of the array is connected together to form an access line, such as a word line. Columns of the array include strings (often termed NAND strings) of memory cells connected together in series, e.g., source to drain, between a pair of select lines, e.g., a source select line and a drain select line. The source select line includes a source select gate at each intersection between a NAND string and the source select line, and the drain select line includes a drain select gate at each intersection between a NAND string and the drain select line. Each source select gate is connected to a source line, while each drain select gate is connected to a data line, such as column bit line.
In order for memory manufacturers to remain competitive, memory designers are constantly trying to increase the density of memory devices. One way to increase the density is by using multilevel memory cells (MLCs). Multilevel memory cells take advantage of the analog nature of a traditional flash memory cell by assigning a respective data value (e.g., as represented by a bit pattern) to each of a plurality of program levels, e.g., to each of a plurality of distinct ranges of threshold voltages (Vt) that can be stored on the memory cells. This technology permits the storage of two or more bits per memory cell, depending on the quantity of program levels, and the stability of threshold voltages during the lifetime operation of the memory cells. The number of program levels, which are sometimes also referred to as threshold-voltage-distribution windows, used to represent a bit pattern of N bits may be 2N.
Each program level may be characterized by a corresponding distinct range of threshold voltages and a threshold voltage (e.g., that may be referred to as a nominal threshold voltage) at the center of the program level (e.g., the center of the range). A margin (e.g., a certain number of volts), such as a dead space, may separate adjacent program levels, e.g., to facilitate differentiating between data values.
With time (e.g., after a certain retention time), the range of threshold voltages of memory cells programmed to a particular program level may spread out as individual memory cell threshold voltages shift (e.g., due to capacitive coupling, read disturb, charge leakage, etc.) so that the range of threshold voltages corresponding to the program level becomes wider than when initially programmed. In particular, the amount of spreading may be greater at higher program levels (e.g., program levels having higher nominal threshold voltages), resulting in wider ranges at the higher program levels. The wider ranges may cause the higher program levels to be closer together with time, such that the margins between these program levels become narrower. This can cause difficulties when trying to differentiate between possible data values of a memory cell.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternatives to programming memory cells.